1. Field of the Invention
This invention relates to the production of visual displays of time dependent measured quantities, for example, analog physiological signals. The invention is concerned particularly with the display by a cathode ray tube (CRT) of high frequency signals with minimum information, which display is produced on the basis of intermittently obtained values or samples which represent successive amplitude conditions or values of the signals. The invention features the use of digital techniques and is characterized by uniformity of brightness of the display and in a requirement for minimum memory.
2. Description of the Prior Art
Analog and digital apparatus for making visible graphs of analog time dependent measured quantities are known in the prior art. One such known form of analog apparatus is disclosed in the copending application bearing Ser. No. 550,984, filed Feb. 19, 1975 by Peter R. Lowe and Tommy N. Tyler. That apparatus yields faithful reproduction of high frequency signals notwithstanding the intermittent nature of the sampled data used as the information from which the display is produced. In effecting such display, that apparatus produces a succession of parallel line segments representative of the minimum to the maximum excursion of the data or input signal from a sampling period preceding the display period. This result is achieved by the use of a pair of amplitude-detector type circuits which are arranged (a) to store the minimum and maximum excursion of an input analog signal, (b) to supply their respective outputs to comparator circuits, and (c) to be reset to zero upon command. The said circuits are operative in complementary manner, during successive alternate periods, to store the minimum and the maximum values of the data signal and to present the stored values to the comparators for unblanking a CRT. Each such period corresponds to the duration of a sweep of the cathode ray tube beam. As a result of alternating between the amplitude-detector circuits, the minimum and maximum values of the data signal during each sweep and retrace periods of the CRT are always detected and stored by one or the other of the detectors. Transient peak and valley information contained in the data signal, including signal envelopes of high frequency signals, is displayed, and the loss of data is minimized. That apparatus, however, is a recording apparatus in which the CRT is operated in a so-called line scan mode.
A form of digital CRT display apparatus for making visible analog time dependent quantities is shown in U.S. Pat. No. 3,653,027 granted on Mar. 28, 1972 to David W. Scheer. That apparatus includes an analog to digital converter as well as a CRT display device. The memory and the CRT display device are synchronized so that a bright spot representative of the information contained in one location of the memory is produced on the CRT display device for each shifting of information in the memory. New information may be made to replace existing information in the memory at a rate less than the shifting rate of the memory whereby the display trace appears to move across the viewing of the screen of the CRT with a constant brightness. With that prior digital apparatus, as long as the analog signal or signals being measured are slewing in one direction or the other, there is little or no error in the graphic representation of the signal or signals. If the signals should undergo a very rapid or abrupt change of direction between sampled periods, however, significant peak errors can occur, in that such peaks will not be indicated or recorded. Thus, the prior digitial display apparatus described is inadequate for displaying abruptly changing or transient data, or for displaying signal envelopes of high frequency signals.